This invention relates to the processing of phosphoric acid, and, more particularly, to the preparation of phosphate values and removal of impurities from impure phosphoric acid.
Phosphoric acid is prepared by several industrial methods. In one, the "wet process", mined ores bearing phosphate values are contacted with concentrated sulfuric acid, producing a somewhat diluted phosphoric acid. The phosphoric acid is subsequently concentrated and used as the starting material in a number of further processes to produce phosphates and phosphate-containing products.
In the wet process, impurities from the ore are dissolved into the phosphoric acid by the concentrated sulfuric acid. These impurities include a wide variety of elements and compounds, such as heavy and light metals, halogens, sulfates, and the like. The impurities variously affect subsequent processing operations and uses. In some cases, the presence of the impurity adversely affects the process itself, as in forming sludges or the like that tend to clog machinery. In other cases, such as phosphates used in food preservation, the impurities themselves are unacceptable whether or not they interfere with processing.
There are numerous approaches for removing impurities from impure phosphoric acid, involving steps such as chemical extraction with a solvent, oxidation, precipitation, and the like. The choice of a particular purification process depends upon the predominant impurities, the intended use of the acid, and the availability of conditions favorable to a particular type of processing.
In one example of a purification process, a calcium salt is added to the dilute impure acid to precipitate fluorides and sulfates, which are removed by filtration or settling. The acid is then contacted to an organic extractant, typically an alcohol or an ether, to remove metallic impurities into the extractment. The impurities are stripped from the extractant, which is recycled for further use. Such extraction typically requires 3 to 7 stages of extraction to be efficient. After extraction, the acid is concentrated to about 40 percent phosphates, and further calcium salts are added to precipitate more fluorine. Finally, the acid is concentrated to a salt.
This processing is time consuming and expensive, particularly in its need for multiple stages of extraction. Multiple contacting tanks and sets of stripping reactors are required. These facilities must be made of expensive corrosion resistant materials, and are consequently expensive.
There is a continuing need for an approach to obtaining purified phosphate values from impure phosphoric acid, such as that produced by the wet process. Such an improved process should be effective and economical to operate, and preferably have a low capital cost. The present invention fulfills this need, and further provides related advantages.